Ribosome

[[Image:ribosome_subunits.png|frame|Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). Side and front view. (1) Head. (2) Platform. (3) Base. (4) Ridge. (5) Central protuberance. (6) Back. (7) Stalk. (8) Front.]]
A ribosome is an organelle composed of RNA and ribosomal proteins. It translatesmRNA into a polypeptide chain (e.g., a protein).
They can be thought of as a factory that builds a protein from a set of genetic instructions. Ribosomes are found in the cytosol (the internal fluid of the cell) of all cells.

The structure and function of ribosomes, and their attendant molecules, known as the translational apparatus, has been of ongoing research interest since the mid 20th century on through the early 21st century.
A triennial conference is held to discuss the ribosome. In 1999, the conference was held in Helsingør, Denmark. The 2002 conference was held in Queenstown, New Zealand [1].

The ribosomal subunits of prokaryotes and eukaryotes are quite similar. However, prokaryotes use 70S ribosomes, each consisting of a (small) 30S and a (large) 50S subunit, whereas eukaryotes use 80S ribosomes, each consisting of a (small) 40S and a bound (large) 60S subunit.[The unit S means Svedberg units, a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle. Svedberg units are not additive - two subunits together can have Svedberg values that do not add up to that of the entire ribosome.]

(AUG) and ends with a stop codon (UAG).]]

In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (the 5' end of the mRNA). The ribosome uses tRNA (transfer RNAs which are RNA molecules that carry an amino acid and present the matching codon, according to the genetic code, to the ribosome) which matches the current triplet on the mRNA to append an amino acid to the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually, several ribosomes are working parallel on a single mRNA.